Process of preparing magnetic recording elements containing transparent recording layer

ABSTRACT

A process of preparing magnetic recording elements containing substantially transparent magnetic recording layer which exhibit excellent magnetic and reproducing characteristics is as follows: 
     (a.) forming a substantially homogeneous dispersion of acicular magnetizable particles in a medium comprising a solution of substantially transparent binder in solvent using particles having an average width of less than about 0.06 micron and an average length up to about 1 micron; the concentration of the binder being at least about 10 parts per 100 parts, by weight, of the particles; up to about 30 parts, by weight, for particles having an average length of at least about 0.06 micron and up to about 40 parts, by weight, for particles having an average length of less than about 0.06 micron, 
     (b.) coating a support with the dispersion in an amount sufficient to provide a final dry layer thickness up to about 5 microns, 
     (c.) removing substantially all solvent from the layer and 
     (d.) treating the layer containing particles in which the average length is at least about 0.06 micron using one or both of the following process steps, 
     (1.) compacting the layer while it is in a malleable state to reduce its thickness, 
     (2.) imbibing into the layer a substantially transparent liquid having a refractive index that is substantially the same as that of the binder.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to magnetic recording elements and processes forpreparing such elements. In one of its aspects, this invention relatesto magnetic recording elements having magnetic recording layers that aresubstantially transparent to visible light while exhibiting excellentmagnetic recording and reproducing characteristics, and also toprocesses for preparing such elements.

2. Description Relative To The Prior Art

Conventional magnetic elements that are used for recording sounds orimages are generally opaque to visible light regardless of the nature ofthe magnetic particles used in such elements. However, it is known thatmagnetic layers formed from magnetic iron oxide dispersed in a binderwill transmit infrared radiation. For example, U.S. Pat. No. 2,950,971,issued Dec. 17, 1957, describes an element having multiple sound tracksand comprising a transparent support coated successively with (1) anoptical sound track capable of modulating infrared and (2) a magneticsound track that contains magnetic iron oxide dispersed in a binder anduniformly transmits infrared. The infrared radiation is transmittedthrough the magnetic sound track to reproduce a previously exposed andprocessed optical sound track. The element described in this patent maybe a motion picture film and the magnetic sound track does not coverthat portion of the film used in the projection of images.

As shown in French Pat. No. 1,227,788, granted Mar. 7, 1960 and CanadianPat. No. 686,172, issued May 12, 1964, a magnetic recording layer may betransparent to visible light when it is very thin and contains a verylow concentration of magnetizable particles such as gamma ferric oxide.According to these patents such a layer is coated over a layercontaining descriptive material which allows a user to simultaneouslyhear and see certain subject matter. However, as pointed out in thesepatents, the electromagnetic characteristics i.e. the magnetic recordingand reproducing characteristics of such a layer are inferior to those ofconventional magnetic layers as a result of the very low concentrationof magnetizable particles.

U.S. Pat. No. 3,782,947, issued Jan. 1, 1974, discloses a photographicproduct which carries magnetizable particles that are uniformlydistributed across the image area of the product. The particledistribution and size(s) are so designed that the compositegranularities of the photographic and magnetic recording media are suchthat the magnetic distribution is essentially transparent in aphotographic sense. According to this patent, the photographic image canbe viewed via the magnetic distribution, and the magnetic distributioncan be employed for recording and playback information.

It is evident that an element containing a magnetic layer which combinestransparency to visible light with magnetic recording and reproducingcharacteristics which are comparable to conventional opaque magneticlayers would represent an advance in the prior art. Likewise, it isevident that it would be desirable to obtain such transparency withoutthe need for matching the granularity of a magnetic medium to that of aphotographic medium, as in U.S. Pat. No. 3,782,948. The process forpreparing such elements would, of course, clearly provide an advance inthe prior art.

SUMMARY OF THE INVENTION

In accordance with this invention we have provided a magnetic recordingelement containing a magnetic recording layer having a combination offeatures as described herein, including a relatively high concentrationof magnetizable particles with respect to binder, is that transparent tovisible light and also exhibits magnetic recording and reproducingproperties that are of the same order as those of conventional magneticlayers that are opaque to visible light. As pointed out in detail in thefollowing specification and claims, magnetic recording elementscontaining such layers are obtained using specific process steps thatare combined to bring about this unique result. By practicing thisprocess, there is obtained a magnetic recording layer exhibiting acharacteristic total transmission of at least about 20 percent forvisible light having a wavelength of 632.8 nm (emission of a helium-neonlaser) and a ratio of direct transmission to total transmission of atleast 50 percent at that same wavelength. The total transmission of alayer consists of light which passes through the layer by directtransmission (also referred to in the prior art as speculartransmission) and by diffuse transmission.

The present invention includes a magnetic recording element comprising asupport and a transparent magnetic recording layer having a thickness upto about 5 microns. This layer contains acicular, magnetizableparticles, i.e. needle-like particles, having an average width of lessthan about 0.06 micron and an average length up to about 1 micron. Theseacicular particles are substantially homogeneously dispersed in a mediumthat comprises a binder and has a refractive index which issubstantially the same throughout the thickness of the transparentmagnetic recording layer. The concentration of this binder is at leastabout 10 parts per 100 parts, by weight, of the magnetizable particles;up to about 30 parts, by weight, for particles having an average lengthof at least about 0.06 micron and up to about 40 parts, per 100 parts,by weight, for particles having an average length of less than about0.06 micron. As previously indicated, the magnetic recording layer has atotal transmission of at least 20 percent for visible light having awavelength of 632.8 nm and a ratio of direct transmission to totaltransmission at this wavelength of at least 50 percent. Often this totaltransmission is at least 60 or 70 percent, or more while the ratio ofdirect to total transmission is frequently considerably above 50percent, e.g. 60, 80, 90 percent or even greater.

The present invention also includes a process for preparing an elementcontaining a magnetic recording layer which combines excellent magneticrecording and reproducing characteristics with transparency, whichprocess comprises:

(a.) forming a substantially homogeneous dispersion of acicularmagnetizable particles in a medium comprising a solution ofsubstantially transparent binder in solvent. These particles have anaverage width of less than about 0.06 micron and an average length up toabout 1 micron, the concentration of said binder being at least about 10parts per 100 parts, by weight, of said particles; up to about 30 parts,by weight, for particles having an average length of at least about 0.06micron and up to about 40 parts, by weight, for particles having anaverage length of less than about 0.06 micron,

(b.) coating a support with this dispersion in an amount sufficient toprovide a final dry layer thickness up to about 5 microns,

(c.) removing substantially all solvent from the layer and

(d.) treating the layer containing particles having an average length ofat least about 0.06 micron using one or both of the following processsteps,

(1.) compacting the layer while it is in a malleable state to reduce itsthickness,

(2.) imbibing into the layer a substantially transparent liquid having arefractive index that is substantially the same as that of the binder.As previously indicated, the magnetic recording layer prepared accordingto this process has a total transmission of at least 20 percent forvisible light having a wavelength of 632.8 nm and a ratio of directtransmission to total transmission at said wavelength of at least 50percent.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE in the drawing shows the relationship of the totaltransmission to diffuse transmission, as a function of wavelength, innanometers, for a transparent magnetic recording layer obtained by thepractice of this invention. It can be seen from the curve set forth inthe drawing that such layers exhibit unexpectedly low absorption in thevisible region of the spectrum, particularly in the green and redregions.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The magnetizable particles dispersed in the transparent recording andplayback layers of this invention are acicular or needle-likemagnetizable particles. They have an average width (minor axis) of lessthan 0.06 micron, often less than about 0.05 micron and frequently inthe range of about 0.02 to 0.05 micron. The average length (major axis)of these particles is up to about 1 micron, often up to about 0.8 micronand frequently in the range of about 0.2 to 0.6 micron. Suitableacicular particles are ferro- and ferri-magnetic particles that have atleast ten percent preferably fifty percent or greater transmission ofvisible radiation, particularly visible radiation in the region of 632.8nm. Typical acicular particles of this type include, for example,particles of ferro- and ferri-magnetic iron oxides such as the browngamma ferric oxide, complex oxides of iron and cobalt, various ferritesand the like. Acicular gamma ferric oxides or ferrous ferric oxides,which can be undoped or can contain doping metal ions, are particularlyuseful in this invention. The particles can be doped with one or moreions of a polyvalent metal such as cobalt, nickel, zinc, manganese,chromium, or the like. The concentration of dopant ion employed issubject to variation, depending upon such things as the size of themagnetizable particles. However, dopant levels in the range of about 1to 6 percent, by weight, often about 1 to 3 percent, by weight,particularly with cobalt ion, are suitable.

The magnetizable particles, present in the elements of this invention,are substantially homogeneously dispersed in a medium that comprises abinder. In practicing the invention, a rather large amount (by weight)of acicular magnetizable particles is used with respect to this binder.This results in a magnetic recording layer which has a high density,such density being the weight of magnetizable particles per unit volumeof the layer. It is the ratio of the weight of magnetizable particles,expressed in milligrams, per volume of magnetic recording layer,expressed in cubic millimeters. Due to their high density, the magneticrecording layers of this invention, exhibit excellent magnetic recordingand reproducing properties.

The concentration of the binder employed in dispersing the magnetizableparticles depends upon the average length of the particles, a largerconcentration of binder being used with the shorter particles. Theconcentration of this binder is at least about 10 parts per 100 parts byweight, of the magnetizable particles; up to about 30 parts, by weight,for particles having an average length of at least 0.06 micron and up toabout 40 parts per 100 parts, by weight, for particles having an averagelength of less than about 0.06 micron. Since the average width of themagnetizable particles employed in the practice of this invention isless than about 0.06 micron, it can be seen that a larger concentrationof binder (10 to 40 parts) is employed with magnetizable particleshaving two of its dimensions, i.e. both average width and averagelength, less than about 0.06 micron. The concentration of binder in suchcase can, of course, be less than 40 parts, by weight, and is often inthe range of about 10 to 25, or even 10 to 15 parts by weight, per 100parts of magnetizable particles. When longer magnetic particles, i.e.those having an average length of at least about 0.06 micron, areemployed in the practice of this invention, the concentration of binderis in the range of about 10 to about 30, often about 10 to about 15 or25, parts by weight, of binder per 100 parts, by weight, of magnetizableparticles. The acicularity, i.e. the ratio of length to width, of themagnetizable particles employed in practicing this invention is subjectto wide variation. Generally, the acicularity of such particles is atleast 2 and often 15 or more. For particles having both an average widthand length up to about 0.06 micron, and particularly gamma ferric oxide(undoped or doped) a suitable acicularity is in the range of about 2 to10. For particles having an average width below 0.06 micron and anaverage length up to 1 micron, e.g. an average length of about 0.1 to0.4 or 0.5 micron, and often about 0.2 micron, a suitable acicularity isin the range of about 5 to 40, generally about 10 to 40. Preferredparticles of this size are particles of gamma ferric oxide (undoped ordoped).

The binders that can be used in the practice of this invention includeany of the substantially transparent binders well known for themanufacture of magnetic recording layers. Typical binders are polymericbinding agents such as copolymers of vinyl acetate with vinyl chloride,copolymers of vinylidene chloride with acrylonitrile, copolymers ofacrylic and/or methacrylic esters, polyvinylbutyral, copolymers ofbutadiene with styrene, terpolymers of acrylonitrile, vinylidenechloride and maleic anhydride, cross-linked or non-cross-linked,homopolymers or copolymers such as polyamide, polyurethanes, polyesters,and the like, as well as mixtures of these binders. Good results can beobtained with a copolymer of vinyl acetate with vinyl chloride,partially hydrolyzed, and possibly cross-linked by an isocyanate orsimilarly reactive constituent, or by using polyurethanes or a mixtureof these binders.

In forming the magnetic recording layers of this invention, themagnetizable particles are homogeneously dispersed in a medium whichcontains one or more of the substantially transparent binders describedpreviously, and a solvent for the binder. The dispersing medium can alsocontain transparent addenda, for example, plasticizers such as tricresylphosphate or dioctyl phthalate or lubricants such as carbonic acid mixedesters, as exemplified by ethyl cetyl phosphate and described in FrenchPat. No. 2,094,663 and like addenda. In the final layer the medium has arefractive index which is substantially the same throughout thethickness of the magnetic recording layer. As pointed out hereinafter,this medium can contain discrete, nonmagnetizable voids, but such voidscan be compressed or filled with a liquid whose refractive index issubstantially the same as the binder employed in the medium. As aresult, the medium will then have the substantially unchanged refractiveindex referred to herein and set forth in the claims.

After the dispersion of acicular magnetizable particles in a solution ofbinder in solvent is formed, it is coated onto a support in an amountsufficient to provide a final dry layer thickness up to about 5 microns,generally up to about 4 microns and preferably about 1 to 3 microns. Thedispersion can be coated directly on the support or it can be coatedover other layers using any process suitable for this purpose,including, for example, those described in the "Encyclopedia of PolymerScience and Technology", John Wiley and Sons, 1965, Volume 3, pages765-833. Substantially, all solvent is then removed from the layer byany suitable means.

In those instances where magnetizable particles having a length of atleast 0.06 micron are used in the magnetic recording layer it is givenat least one further treatment. In this case the layer is compactedwhile it is in a malleable state to reduce its thickness and/or asubstantially transparent liquid having a refractive index that issubstantially the same as the binder for the magnetizable particles isimbibed into the layer. One method of compacting the magnetic recordinglayer is to calender it by passing the element containing this layer(e.g. before the binding agent has lost its thermoplastic properties)between very smooth, hard steel rolls or between a very smooth, hardsteel roll and a cotton roll with the steel roll in contact with themagnetic recording layer. The magnetic recording layer is advantageouslysubjected to calendering several times until the amount of lightdiffused by the layer is constant, and preferably, the layer is heatedto facilitate compacting. If calendering is to be used, it is generallydesirable to select a binder that has a relatively low glass transitiontemperature (Tg) since such a binder is particularly suited tocalendering. In calendering the layer, it is generally sufficient toheat the steel roll to a temperature at least 10° C. above the Tg of thebinder. Calendering the magnetic recording layer is very desirablebecause this generally provides a layer of very high density, i.e., avery high weight of magnetizable particles per unit volume. It isdesirable to calender the magnetic recording layer to a thickness belowabout 4 microns, and in most cases, to a thickness in the range of about1 to 3 microns since the transparency of the layer increases as thethickness of the layer decreases.

Compacting the magnetic recording layer while it is still in themalleable state using means such as calendering provides an extremelysmooth surface which has a favorable effect upon the transparency of thelayer. This surface smoothness can be expressed in terms of "percentcontact area". This "percent contact area" is determined with relationto a reference surface consisting of the hypotenuse surface of atransparent prism. The value of an incident luminous flux directed fortotal reflection from the hypotenuse is equal to φ. The flux reflectedby the hypotenuse surface is, therefore, equal to φ but is reduced andbecomes equal to φ' when one places an absorber in optical contact withthe hypotenuse. A sample of the magnetic element being measured, whencontacted under controlled pressure against the surface (with theoutermost magnetic recording layer of the element in contact with thesurface) produces light absorption that increases as the smoothness ofthe magnetic recording layer increases. The "percent contact area" isequal to ##EQU1## The value of the "percent contact area" increases withthe flatness of the surface of the magnetic recording layer, i.e., withthe surface smoothness of the layer. The magnetic recording layersobtained by compacting the surface, as described herein, exhibit asurface smoothness determined as "percent contact area" of at leastabout 70 percent and often in the order of 80 percent or more. Such highsurface smoothness provides intimate physical contact between arecording or reproducing magnetic head and the magnetic recording layer.This improves signal reproduction and is particularly significant invideo recording where short wavelengths are used.

As previously indicated, the transparency of magnetic recording layerscontaining acicular magnetizable particles having a length of at least0.06 micron can be improved by imbibing into it a substantiallytransparent liquid having a refractive index that is substantially thesame as the binder in the layer. This liquid is believed to partially orcompletely fill discrete, nonmagnetizable voids or pores that arepresent in the medium in which the acicular magnetizable particles aredispersed. These voids or pores are non-solid, contain entrapped air andare formed within the medium when the binder solvent is removed duringdrying of the magnetic recording layer. Low viscosity oils such aslinseed oil are examples of satisfactory materials that can be imbibedinto the magnetic recording layers according to this invention.

As pointed out herein, the compacting and imbibing treatments are usedwith magnetic recording layers containing acicular magnetizableparticles having a length of at least 0.06 micron, i.e. the longerparticles employed in practicing this invention. However, each of thesetreatments can be used, alone or in combination, with magnetic recordinglayers containing shorter acicular magnetizable particles, i.e., thosehaving a length less than about 0.06 micron. It is possible to obtain amagnetic recording layer having the transparency characteristics (totaltransmission and ratio of direct transmission to total transmission)described herein without submitting such a layer to these subsequenttreatments after coating and drying. However, it is generallyadvantageous to use one or both of these subsequent treatments tofurther improve the transparency characteristics of a magnetic recordinglayer containing the shorter acicular magnetizable particles.

In practicing this invention, transparent liquids such as linseed oilcan be imbibed into the magnetic recording layers using any methodsuitable for this purpose. For example, the magnetic element can simplybe dipped in the liquid or the liquid can be applied to the magneticrecording layer using conventional coating techniques such as hoppercoating, spray coating and the like.

To obtain good magnetic and optical properties the dispersion ofacicular magnetizable particles in a medium comprising a solution ofbinder in solvent used in forming the magnetic layers of this inventionmust be homogeneous since these magnetic particles tend to formagglomerates upon coating. Such agglomerates are detrimental to lighttransmission and are known to provide undesirable magneticdiscontinuities and result in noise. To avoid the formation of theseaggregates, coating dispersions, including those employed in thepractice of this invention, are generally subjected to shear afterpreparation and prior to coating. Many suitable techniques are knownwhich will avoid agglomeration of magnetic particles and/or theformation of undesirable binder "slugs". Any of these techniques, manyof which are known in the paint industry, can be employed to achieve thesubstantially homogeneous dispersion of magnetizable particles in thedispersing medium used in this invention. In many cases, a dispersingagent such as a fatty acid amine derivative will facilitate dispersionof the magnetizable particles.

Suitable solvents that can be employed in the medium used to dispersethe magnetizable particles include organic materials such as methylethyl ketone, methyl isobutyl ketone, ethyl acetate, butyl acetate,cyclohexanone, butyl alcohol, methylene chloride, tetrahydrofurandioxane, dimethylformamide and the like as well as mixtures thereof.

After the dispersion of acicular, magnetizable particles is formed it iscoated onto a support. The dispersion can be coated directly on thesupport or it can be coated over or under other layers to form themagnetic recording layer. The acicular, magnetizable particles can bealigned in the still wet fluid layer, i.e. the layer still containingsolvent, after it is applied to the support. This alignment can beaccomplished by conventional means such as applying a magnetic field tothe magnetic recording layer while it still contains solvent and theacicular particles are sufficiently movable therein to be aligned. Asuitable magnetic field strength for this purpose is generally about2000 Oersteds.

The magnetic dispersion described herein can be applied to asubstantially transparent support or to an opaque support. Thetransparency or opaqueness of the support depends upon the use theelement will be put to, e.g. transparent supports are used for motionpicture film in which the transparent magnetic recording layer of thisinvention provides a means for recording and reproducing audioinformation. The transparent magnetic recording layers of this inventioncan also be coated over a layer which contains descriptive material andis coated on an opaque support. This allows a user to view thedescriptive material by reflected light and to simultaneously hearsubject matter recorded in the magnetic recording layer. Clearly, themagnetic dispersions described herein can be applied to a wide varietyof nonmagnetizable supports, including discs, belts, paper or film, andthe like. Suitable supports can be subbed by known methods, aregenerally flexible and typically include such materials as cellulosenitrate film, cellulose acetate film, polyvinyl acetal film, polystyrenefilm, polyester such as poly(ethylene terephthalate) film, which can bebiaxially or asymmetrically stretched, polycarbonate film, and relatedfilms or resinous materials, as well as papers, metals such as aluminumor brass, and the like.

The elements of this invention can contain radiation-sensitiveimage-forming layers in addition to the transparent magnetic recordinglayers. Such image-forming layers can be photosensitive layers, asexemplified by layers containing photographic silver halides such assilver chloride, silver bromide, silver bromoiodide, silverchlorobromide, and the like, or they can contain non-silverphotosensitive materials such as diazo compounds, inorganic and organicphotoconductors, photosensitive resins, bichromated colloids, and thelike. Suitable radiation-sensitive image-forming layers are those whichprovide black-and-white or color images.

The transparent magnetic layer and the radiation-sensitive image-forminglayers can be coated on either surface of the support. These layers canbe superposed on one surface of the support. According to one embodimentof this invention, a transparent support is coated on one of itssurfaces with a radiation-sensitive image-forming layer and, on theother surface, with a transparent magnetic recording layer. According toanother embodiment, a transparent support is coated successively, on oneof its surfaces with a radiation sensitive image-forming layer and atransparent magnetic recording layer. Such a product can also comprise,on the other surface, a second transparent magnetic recording layer.

An element of this invention which comprises a radiation sensitiveimage-forming layer and at least one transparent magnetic recordinglayer has a high capacity for information. It also has severaladvantages over conventional photographic films carrying one or moremagnetic tracks or narrow and opaque magnetic stripes which record soundor other information such as latitude, longitude, altitude, etc. Forexample, in certain motion picture films, a narrow stripe containingmagnetic iron oxide in a binder extends along one and/or two edges ofthe film surface so that one may record sound or other informationmagnetically. After development of the exposed film, light istransmitted through the area of the film not carrying the magnetictrack. This kind of film is satisfactory when its width is small and itis used in the form of spools of relatively small diameter. However,with spools of larger diameter, nonuniform winding results from theincreased thickness of these tracks, particularly with large films ofthe type used in aerial photography. It is obvious that elements madeaccording to this invention are not subject to this disadvantage becausethe transparent magnetic recording layer can be uniformly coated acrossthe image areas of the film.

The magnetic recording elements of this invention can be used with theusual magnetic recording and reading equipment. Where the element istransparent, it can also be used with a system that takes advantage ofthis transparency. For example, when a linearly polarized luminous beamcrosses a transparent magnetic layer, the polarization plane undergoes arotation proportionate to the intensity of magnetization of the magneticmaterial in the layer. This is a result of the Faraday effect. Thus, onecan read a transparent magnetic recording element of the type disclosedherein by eliminating any contact between the element and the readingmember, by making use of the Faraday effect, e.g. as described in FrenchPat. No. 2,248,754. In this case, the support used in the element ofthis invention is advantageously an optically inactive transparent filmsuch as cellulose ester film, as exemplified by cellulose triacetatefilm.

The intended use of the magnetic recording elements of this inventionwill determine the specific coercivity of the acicular magnetizableparticles employed in the magnetic recording layers. The coercivity ofthese particles is subject to wide variation, but it is generally atleast about 140 Oe, often about 140 to 300 Oe, particularly with undopedacicular gamma ferric oxide, up to about 1000 or 2000 Oe or even higher,particularly for doped acicular gamma ferric oxide such as cobalt dopedgamma ferric oxide.

Unlike prior art transparent magnetic recording layers such as describedin French Pat. No. 1,227,788 and Canadian Pat. No. 686,172, thetransparent magnetic recording layers of this invention have a highdensity (ratio of the weight of magnetizable particles, in mg., pervolume of magnetic layer, in mm³), which provides excellent recordingand reproduction properties. This density is at least equal, and isgenerally superior to those of conventional nontransparent magneticrecording layers; the density of the latter generally being in the rangeof about 1.5 to 2. As illustrated by the following examples, ourtransparent magnetic recording layers containing gamma ferric oxide(average particle length of about 0.2 micron) having a density of 2 orgreater.

In the following examples the magnetic properties of a magneticrecording layer of this invention are determined by means of itshysteresis loop which is obtained in a conventional manner. As is wellknown in the art, the hysteresis loop results when the magneticinduction B is plotted in response to an applied magnetic intensity H ina magnetic substance, as H is varied cyclically between equal andoppositely directed values. The coercivity (H_(c)) provides informationrelating to the magnetic performance of the layer (optimum writing orbias current). The residual flux of the layer (φ_(r)) from whichresidual flux per micron of layer thickness is readily determined,constitutes a measure of the potential qualities of the magneticrecording layer. It will be appreciated that these qualities are betterfor a layer having a greater residual flux per unit of thickness.

As illustrated by the following examples, the optical characteristics ofthe magnetic recording layers of this invention, can be determined bymeans of a conventional spectrophotometer. The spectrophotometerprovides data for curves of the type set forth in the drawing whichcurves gives the amounts of directly transmitted light (T_(D)) and ofdiffuse light (T_(S)) as a function of wavelength, for a specificmagnetic recording layer and support. Since the supports employed aresubstantially transparent, these amounts are essentially those of themagnetic recording layer. The total amount of transmitted light is equalto T_(D) +T_(S). A quality factor for a given magnetic recording layerin terms of the ratio R, expressed in percent, is determined accordingto the following: ##EQU2## The transparent quality of the magneticrecording layers of this invention improves as T_(S) decreases, i.e., asR approaches 100.

The invention is further illustrated by the following examples of itspractice. Example 9 illustrates the results obtained when cubic gammaferric oxide particles are substituted for the acicular magnetizableparticles used in the magnetic recording layers of this invention andwas included for comparison purposes.

EXAMPLE 1

150 g. of gamma ferric oxide particles (average length about 0.05micron, acicularity of 2 to 3) and a collodion obtained by dissolving 15g. of a partially hydrolyzed vinyl chloride-vinyl acetate copolymer(commercially available under the trade name VAGH from Union CarbideCorp.) in 300 ml of methylisobutylketone were charged to a 1 literporcelain ball mill containing 1 kg. of glass beads 7 mm in diameter.

After milling for 72 hours a collodion containing 7.5 g. of partiallyhydrolyzed vinyl chloride-vinyl acetate copolymer and 4.5 g. oftricresyl phosphate in 60 ml of methylisobutylketone were added to themill. Milling was continued for 24 hours to homogenize the dispersion.

The iron-oxide dispersion was separated from the balls, filtered throughfritted bronze having a pore size of 5 microns and then degassed invacuum. The dispersion was coated onto 25 micron thick poly(ethyleneterephthalate) support in an amount sufficient to provide a layer 2microns thick in the dry state. A sample of the element was calenderedby passing it through two steel cylinders at 80° C. using a linearpressure of 350 kg/cm. The surface of the resulting magnetic layer had apercent contact area of 80%.

Two other samples of the element, one calendered and one not were dippedin linseed oil.

The total transmission and diffuse transmission of each of these samplesat a wavelength of 632.8 nm was determined using a Cary 14 Spectrometer.The wavelength of 623.8 nm is the wavelength emitted by a helium-neonlaser. The total transmission through the magnetic recording layer andthe support and the ratio between the direct transmission and the totaltransmission for each of the samples was as follows:

                  TABLE I                                                         ______________________________________                                                                         Direct                                                              Total     Transmission                                                        Trans-    Total                                                    Thickness  mission   Transmission                                 Sample      of layer (μ)                                                                          (Percent) (Percent)                                    ______________________________________                                        Noncalendered                                                                             2          60.5      53.5                                         Noncalendered                                                                 + oil       2          72.7      81.8                                         Calendered  1.5        68.5      84                                           Calendered + Oil                                                                          1.5        77        91.9                                         ______________________________________                                    

As shown in the above table, the sample of the noncalendered tape had asatisfactory transparency. It can be seen that the transparency wasincreased substantially by calendering or by impregnation of themagnetic recording layer with linseed oil and the best results wereobtained by calendering followed by impregnation with oil.

The magnetic properties of the coated elements of this inventionobtained in this and the following examples were determined by means oftheir hysteresis loop, as previously described herein, using 6.25 mmwide samples. For illustration purposes, these properties were comparedwith those of a commercially available high quality low noise audiotape, i.e. Kodak Magnetic tape C120 LN. The results obtained with thecalendered sample of this example and for the commercial tape are setforth in the following Table II. For comparison purposes, the amount ofiron oxide and the thickness of the magnetic layers in the two elementsis also set forth in Table II. The coercivity, Hc, is expressed inOersteds (Oe) and the residual flux, φr, in webers (Wb).

                  TABLE II                                                        ______________________________________                                                       Thick-                                                                Amount  ness                                                                  Iron Oxide                                                                            of the  Hc     φr φr/μ                                     (g/m.sup.2)                                                                           layer   (Oe)   (Wb)   (Wb/μ)                                ______________________________________                                        Calendered                                                                    Sample   3.2       1.5     150  105.10.sup.-11                                                                       70.10.sup.-11                          Kodak mag-                                                                    netic tape                                                                    C 120 LN 7.8       4.5     290   25.10.sup.-10                                                                       55.10.sup.-11                          ______________________________________                                    

As shown in the above Table, the residual flux per unit thickness of thesample prepared according to this invention is of the same order ofmagnitude as that of the commercial tape.

EXAMPLE 2

A magnetic dispersion was prepared as in Example 1 except that theconcentration of the binder was increased to 40 parts, by weight, per100 parts, by weight, of magnetic iron oxide.

This dispersion was coated onto a 25 micron thick poly(ethyleneterephthalate) support. After drying the transparent magnetic layer hada thickness of 2.4 microns.

A sample of this product was calendered as in Example 1 and a secondsample was imbibed with linseed oil as in Example 1.

The transmission characteristics of these samples were determined asdescribed in Example 1. The results are set forth in the following TableIII.

                  TABLE III                                                       ______________________________________                                                                         Direct                                                             Total      Transmission                                                       Trans-     Total                                                  Thickness   mission    Transmission                                 Sample    of layer(μ)                                                                            (Percent)  (Percent)                                    ______________________________________                                        Noncalendered                                                                           2.4         56         56.1                                         Noncalendered                                                                 + oil     2.4         61.4       71                                           Calendered                                                                              2.3         59         69.2                                         ______________________________________                                    

As in Example 1, transparency of the magnetic recording layer isimproved by calendering or by imbibing linseed oil into the layer.However, there is less improvement in comparison to the sample inExample 1 because the magnetic recording layer of the sample obtained inthis Example 2 contains an amount of binder which is larger than theamount used in Example 1. This renders such a layer less porous and lesssensitive to the effects of calendering and impregnation by oil.

EXAMPLE 3

150 g. of acicular gamma ferric oxide doped with cobalt (Co/Fe₂ O₃ =4.8%by weight) and having an average length of 0.2μ and an acicularity ofabout 8, and a collodion containing 15 g. of partially hydrolyzed vinylchloride-vinyl acetate copolymer in 240 ml of methylisobutylketone werecharged to a 1 liter porcelain ball mill containing 2 kg. of steel balls8 mm. in diameter.

After 72 hours of milling a collodion containing 22.5 g. of partiallyhydrolyzed vinyl chloride-vinyl acetate copolymer, 150 ml ofmethylisobutylketone and 8 ml. of ethyl cetyl carbonate were added andmilling was continued for 24 hours to thoroughly homogenize thedispersion.

After separating the dispersion from the balls, it was filtered throughfritted bronze having a pore size of 5μ and degassed in vacuum. Thedispersion was coated on a 37μ thick cellulose triacetate support. Afterdrying, the element was calendered by passing the magnetic tape betweentwo steel cylinders at 90° C. using a linear pressure of 350 kg/cm.

The magnetic recording layer of this element had a thickness of 3μ,contained 6 mg/m² of iron oxide and had a density of 2 and a surfacesmoothness, expressed as percent contact area, of 72%.

The magnetic properties of the magnetic recording layer were determinedas in Example 1 and are set forth in the following Table IV. Forcomparison, the same properties of Kodak magnetic tape C 120 LN areprovided. The amount of iron oxide and the thickness of the magneticlayers are also provided.

                  TABLE IV                                                        ______________________________________                                               Amount  Thickness                                                             Iron Oxide                                                                            of the   Hc     φr φr/μ                                    (g/m.sup.2)                                                                           layer    (Oe)   (Wb)   (Wb/μ)                               ______________________________________                                        Sample   6         3        980  20.10.sup.-10                                                                        67.10.sup.-11                         Kodak mag-                                                                    netic tape                                                                    C 120 LN 7.8       4.5      290  25.10.sup.-10                                                                        55.10.sup.-11                         ______________________________________                                    

Using a sample of the element prepared according to this Example, totaltransmission and diffuse transmission were determined as in Example 1.The results were as follows:

                  TABLE V                                                         ______________________________________                                                                  Direct                                                            Total       Transmission                                                      Trans-      Total                                               Thickness of the                                                                            mission     Transmission                                        magnetic layer (')                                                                          (Percent)   (Percent)                                           ______________________________________                                        3            24           58                                                  ______________________________________                                    

EXAMPLE 4

A magnetic dispersion was prepared as in Example 3 except that theconcentration of binder was reduced to 20 parts, by weight, per 100parts, by weight of iron oxide, i.e. the total quantity of binder was 30g. rather than 37.5 g.

The dispersion was coated as in Example 3, dried and the totaltransmission and diffuuse transmission of the element determined as inExample 1.

The element was calendered as in Example 3 and the transmissioncharacteristics were again determined. The surface of the calenderedmagnetic layer had a percent contact area of 80%. A sample of thecalendered element was dipped in linseed oil and the transmissioncharacteristics were again determined. The results were as follows:

                  TABLE VI                                                        ______________________________________                                                                          Direct                                                             Total      Transmission                                                       Trans-     Total                                                  Thickness   mission    Transmission                                Sample     of layer(μ)                                                                            (Percent)  (Percent)                                   ______________________________________                                        Non calendered                                                                           5           3          7                                           Calendered 4           20         69                                          Calendered + oil                                                                         4           27.5       84                                          ______________________________________                                    

EXAMPLE 5

150 g. of acicular, gamma ferric oxide, (length of 0.2μ and acicularity6), a collodion obtained by dissolving 22.5 g. of partially hydrolyzedvinyl chloride-vinyl acetate copolymer in 360 ml. ofmethylisobutylketone and 5 g. of glycol caprylate were charged to a 1liter porcelain ball mill containing 1 kg. of ceramic balls 7 mm indiameter.

After milling for 120 hours the dispersion was separated from the balls,filtered on a cartridge of fritted bronze having a pore size of 5μ,degassed in vacuum, and coated on a 23μ thick poly(ethyleneterephthalate) support. The magnetic recording layer was calenderedbetween two steel cylinders at 80° C. using a linear pressure of 350kg./cm. The magnetic layer had a density of 2.1 and a surface having apercent contact area of 80%.

The magnetic properties of the recording layer were determined as inExample 1 and are set forth in the following Table VII. This table alsoprovides layer thickness and iron oxide content. For comparison, thecorresponding values for Kodak magnetic tape C 120 LN are set forth.

                  TABLE VII                                                       ______________________________________                                               Amount                                                                        Iron   Thickness                                                              Oxide  of the    Hc     φr φr/μ                                    (g/m.sup.2)                                                                          layer(μ)                                                                             (Oe)   (Wb)   (Wb/μ)                               ______________________________________                                        Sample   3.2      1.5       131  114.10.sup.-11                                                                       76.10.sup.-11                         Kodak mag-                                                                    netic tape                                                                    C 120 LN 7.8      4.5       290   25.10.sup.-10                                                                       55.10.sup.-11                         ______________________________________                                    

The transmission characteristics of the sample were determined as inExample 1. The total transmission and diffuse transmission were plottedas a function of wavelength, expressed in nanometers. The curve obtainedis set forth in the accompanying drawing.

The magnetic properties of the sample, determined as in Example 1, wereas follows:

                  TABLE VIII                                                      ______________________________________                                                                  Direct                                                            Total       Transmission                                                      Trans-      Total                                               Thickness of the                                                                            mission     Transmission                                        layer (μ)  (Percent)   (Percent)                                           ______________________________________                                        1.5           63          90                                                  ______________________________________                                    

A seen from the above table the magnetic element of this example has aremarkably high transparency.

EXAMPLE 6

A magnetic element was prepared as described in Example 5 except that a100μ thick poly(ethylene terephthalate) support was used. The element,after calendering, had a 1.5μ thick transparent magnetic recording layerwhose properties were comparable to those set forth in Table VIII.

A negative photographic silver bromoiodide gelatin emulsion layer wascoated on the opposite side of the support to provide a photographicelement.

EXAMPLE 7

A magnetic dispersion was prepared as described in Example 5 usingacicular, gamma ferric oxide doped with cobalt (Co/Fe₂ O₃ =1.5%, byweight) and having an average length of 0.2 and an acicularity of about6. The dispersion was coated onto a 23μ thick poly(ethyleneterephthalate) support and calendered as in Example 3. The magneticrecording layer obtained had a density of about 2 and a surface having asmoothness expressed in percent contact area of 80%.

The magnetic properties of the magnetic layer were determined as inExample 1 and are set forth in the following Table IX. This table alsoprovides layer thickness and iron oxide content. For comparisonpurposes, the corresponding values for Kodak magnetic tape C 120 LN areset forth.

                  TABLE IX                                                        ______________________________________                                                       Thick-                                                                Amount  ness                                                                  Iron Oxide                                                                            of the   Hc     φr φr/μ                                    (g/m.sup.2)                                                                           layer (μ)                                                                           (Oe)   (Wb)   (Wb/μ)                               ______________________________________                                        Sample   5.7       2.8     290   383.10.sup.-11                                                                       68.10.sup.-11                         Kodak mag-                                                                    netic tape                                                                    C 120 LN 7.8       4.5     290    25.10.sup.-10                                                                       55.10.sup.-11                         ______________________________________                                    

The transmission characteristics of the sample, determined as in Example1, were as follows:

                  TABLE X                                                         ______________________________________                                                                  Direct                                                            Total       Transmission                                                      Trans-      Total                                                             mission     Transmission                                        Thicknessμ (Percent)   (Percent)                                           ______________________________________                                        2.8           39          72                                                  ______________________________________                                    

EXAMPLE 8

150 g. of acicular, gamma ferric oxide doped with cobalt (Co/Fe₂ O₃=2.5%, by weight) having an average length of 0.3 to 0.4μ and an averageacicularity of about 9, and a collodion obtained by dissolving 15 g. ofpartially hydrolyzed vinyl chloride-vinyl acetate copolymer in 320 ml ofmethylisobutylketone were charged to a 1 liter porcelain ball millcontaining 1 kg. of ceramic balls 7 mm. in diameter.

After milling for 120 hours, a collodion containing 7.5 g. of partiallyhydrolyzed vinyl chloride-vinyl acetate copolymer, 100 ml. ofmethylisobutylketone and 4.5 g. of tricresyl phosphate was added to themill.

After milling for 24 hours, the dispersion was separated from the balls,filtered on a cartridge of fritted bronze having a pore size of 5μ andthen degassed in vacuum. This dispersion was coated onto a 25μ thickpoly(ethylene terephthalate) support to obtain a layer 1.8μ thick in thedry state. After drying the layer, a sample of the element wascalendered using the conditions of Example 5. Samples of thenoncalendered and calendered element were dipped into linseed oil toimbibe the oil into the magnetic layers.

The transmission characteristics of the samples was determined as inExample 1. The results were as follows:

                  TABLE XI                                                        ______________________________________                                                                          Direct                                                              Total     Transmission                                                        Trans-    Total                                                               mission   Transmission                                Sample       Thickness(μ)                                                                          (Percent) (Percent)                                   ______________________________________                                        Noncalendered                                                                              1.8        23.6      10.6                                        Noncalendered                                                                 + oil        1.8        38.7      50.2                                        Calendered   1.4        42        58.3                                        Calendered + oil                                                                           1.4        55.2      79.3                                        ______________________________________                                    

As in the preceding examples, there was a substantial increase in thepercentage of total transmitted light and in the directtransmission/total transmission ratio, as a result of calendering andimpregnation with linseed oil.

EXAMPLE 9

Cubic gamma ferric oxide particles having average dimensions of about0.1μ were dispersed and coated and the coated elements were analyzed,all as in Example 1.

The following results were obtained:

                  TABLE XII                                                       ______________________________________                                                                          Direct                                                              Total     Transmission                                                        Trans-    Total                                                    Thickness  mission   Transmission                                Sample       of layer(μ)                                                                           (Percent) (Percent)                                   ______________________________________                                        Noncalendered                                                                              2.4        6.5       0                                           Noncalendered                                                                 + oil        2.4        10.5      9.5                                         Calendered   2.25       12.5      23.2                                        Calendered + oil                                                                           2.25       18        42                                          ______________________________________                                    

As shown by the above table, the magnetic recording layers preparedcontaining the cubic gamma ferric oxide particles were stronglydiffusing. Clearly, such oxides are not suitable for use in transparentmagnetic recording layers.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

We claim:
 1. A process for preparing an element containing a magneticrecording layer which combines excellent magnetic recording andreproducing characteristics with transparency, said processcomprising:(a) Forming a substantially homogeneous dispersion ofacicular magnetizable particles in a medium comprising a solution ofsubstantially transparent binder in solvent,said particles having anaverage width of less than about 0.06 micron and an average length up toabout 1 micron, said particles being ferro- or ferri-magnetic particleshaving a transmission of at least 10 percent for visible light having awavelength of 632.8 nm, the concentration of said binder being at leastabout 10 parts per 100 parts, by weight, of said particles; up to about30 parts, by weight, for particles having an average length of at leastabout 0.06 micron and up to about 40 parts, by weight, for particleshaving an average length of less than about 0.06 micron, (b) coating asupport with said dispersion in an amount sufficient to provide a finaldry layer thickness up to about 5 microns, (c) removing substantiallyall solvent from said layer while forming pores therein and (d) treatingsaid layer when it contains particles having an average length of atleast about 0.06 micron, with at least one of the following processsteps,(1) compacting said layer while it is in a malleable state toreduce its thickness, (2) imbibing into said layer a substantiallytransparent liquid having a refractive index that is substantially thesame as that of said binder, said liquid at least partially filling saidpores in said layer, said magnetic recording layer having a totaltransmission of at least 20 percent for visible light having awavelength of 632.8 nm and a ratio of direct transmission to totaltransmission at said wavelength of at least 50 percent.
 2. A processaccording to claim 1 where said particles have an average length of lessthan about 0.06 micron and an acicularity of about 2 to
 10. 3. A processaccording to claim 1 where said particles have an average length of atleast about 0.06 micron and an acicularity of about 10 to
 40. 4. Aprocess according to claim 1 where said layer is compacted to athickness of about 1 to 3 microns.
 5. A process according to claim 4where said layer is compacted by calendering and has a surfacesmoothness, determined as percent contact area, of at least 80 percent.6. A process according to claim 4 where a substantially transparentliquid having a refractive index that is substantially the same as saidbinder is imbibed into said layer.
 7. A process according to claim 2where said particles are gamma ferric oxide particles or gamma ferricoxide particles doped with cobalt.
 8. A process according to claim 3where said particles are gamma ferric oxide particles or gamma ferricoxide particles doped with cobalt.
 9. A process according to claim 7where said support is a substantially transparent support.
 10. A processaccording to claim 8 where said support is a substantially transparentsupport.